How to spread objects along axis?
Two keyframes will suffice, if the centers of the objects are already spread out in the correct order.


Check the manipulate center points button.

&#8679; Shift RMB select the objects.
Press I and choose Location to keyframe the location.
Move the cursor in the timeline to the second position. I have the dopesheet open to see the created keyframes.
Press S to scale the objects. Because of the Manipulate Center Points only button, the objects will only be moved, but not scaled. You can constrain the movent to a specific axis, by pressing S>X, S>Y or S>Z.
Repeat 3 to insert a second keyframe.